You can not get conclusive evidence for most health topics within a reasonable timeframe. This is not physics, you cannot just repeat experiments dozens of time

You can not get conclusive evidence for most health topics within a reasonable timeframe. This is not physics, you cannot just repeat experiments dozens of times to make sure. Most studies, even those done to actually get medicine approved, are relatively poor. If there is a pandemic unfolding and low Vitamin D is very strongly correlated with severe disease outcome, I do not wait for some study to conclusively prove that Vitamin D is a causal factor. I pop those 4000 IU pills and hope for the best, because the potential upside is far greater than the downside.